1. Field of the Invention
The present invention generally relates to a high frequency heating apparatus of a type utilizing dielectric heating for heating a dielectric material such as, for example, foods and, more particularly, to the high frequency heating apparatus utilizing an inverter power supply designed to convert into a high frequency alternating electric power a direct current electric power obtained by rectifying a commercial electric power.
2. Description of the Prior Art
One typical prior art high frequency heating apparatus will be discussed with reference to FIGS. 1 to 3. Referring first to FIG. 1 showing an electric circuit diagram of an electric power supply circuit used in the prior art high frequency heating apparatus, an electric power from a commercial electric power source 1 is rectified by a rectifier 2 into a direct current electric power which is subsequently supplied through a filtering circuit, including an inductor 3 and a capacitor 4, to semiconductor switching device 7 and also to a resonance circuit including a capacitor 5 and an inductor 6. The illustrated circuit employs a circuit design of a so-called "Isseki-type voltage resonating circuit". The inductor 6 concurrently serves as a primary winding of a transformer which includes, in addition to the primary winding 6, a secondary winding 8 for boosting a voltage applied to the primary winding 6 and a third winding 9 for lowering the voltage applied to the primary winding 6. A high voltage induced in the secondary winding 8 is rectified by a high voltage rectifying circuit 10 into a high direct current voltage. An electric power supply circuit comprising those elements as described above is hereinafter referred to as an inverter power supply 11.
The high D.C. voltage rectified by the high voltage rectifying circuit 10 is applied between an anode and a cathode of a magnetron 12 to excite the latter. A low A.C. voltage induced by the third winding 9 is applied to the cathode of the magnetron 12 to heat the cathode thereof. The magnetron 12 has an outer appearance such as shown in FIG. 2 and has the cathode constituted by a tungsten filament 13. The anode 14 of the magnetron 12 is constituted by a casing for the magnetron 12 and a space 15 between the cathode and the anode is highly evacuated to a substantial vacuum. The cathode 13 and the anode 14 are insulated from each other by means of a ceramic portion 16. The magnetron 12 can be oscillated to generate microwaves when a high voltage of about -4 kilovolts (assuming that the anode 14 is held at zero potential) is applied between the anode 14 and the cathode 13 and, also, the cathode is heated to a predetermined temperature.
Referring still to FIG. 1, a connection between the magnetron 12 and the inverter power supply 11 is carried out in the following manner. The cathode 13, which is a high voltage portion, and a high voltage side of the high voltage rectifying circuit 10 are connected together through an insulated wiring 17, but the anode 14, which is held at the zero potential, and a zero potential side of the high voltage rectifying circuit 10 are connected together through a chassis 18 of the high frequency heating apparatus, which chassis 18 is generally made of metal such as, for example, iron plate. FIG. 3 illustrates a mounting of both of the inverter power source 11 and the magnetron 12 on the chassis 18 of the high frequency heating apparatus. The high frequency heating apparatus so far shown in FIG. 3 comprises an oven-defining structure 19 having a heating chamber and an access opening leading into the heating chamber, a hingedly supported door 20 for selectively opening and closing the access opening, and a control panel 21. The microwaves generated by the magnetron 12 are radiated into the heating chamber of the oven-defining structure 19 to accomplish dielectric heating of, for example, food material placed within the heating chamber. While the cathode 13 which is the high voltage portion of the inverter power supply 11, and the high voltage side of the high voltage rectifying circuit 10 are connected together through the insulated wiring 17, the zero potential side of the high voltage rectifying circuit 10 is connected to the chassis 18 of the high frequency heating apparatus by means of a suitable connecting means 21 such as, for example, wiring, and also with the anode 14 through the chassis 18.
The chassis 18 of the high frequency heating apparatus has a propeller fan assembly 22 rigidly mounted thereon for cooling the magnetron 12 and the inverter power supply 11.
As hereinabove discussed, the prior art high frequency heating apparatus comprises the oven-defining structure, the chassis, the door, the control panel having a plurality of control elements for controlling the high frequency heating apparatus, the magnetron for generating the microwaves, the inverter power supply for driving the magnetron and the fan assembly for cooling both the inverter power supply and the magnetron. An assembly of the prior art high frequency heating apparatus has hitherto been carried out by the following manner. Those component parts described above are individually and sequentially mounted on the chassis by attendant workers and, thereafter, requisite electric connection between the inverter power supply and the control elements in the control panel and requisite electric connection between the inverter power supply and the magnetron are carried out. However, with the prior art high frequency heating apparatus of the above described construction, difficulties have been encountered in implementing the requisite electric connection, requiring a prolonged time to accomplish it. Also, since the inverter power supply, the magnetron and the fan assembly are individually and sequentially mounted on the chassis, an automatic mounting of those component parts is very difficult to accomplish.
In view of the foregoing, an attempt has been made to unite the inverter power supply, which is a microwave generating portion, the magnetron and the cooling means for cooling them into a unitary structure comprising a metallic housing. When they are accommodated in the metallic housing, a cooling system for cooling the magnetron and component parts comprising the inverter power supply can be mounted on a printed circuit board on which those component parts comprising the inverter power supply, and therefore, an electric power necessary to drive the cooling means can be supplied from the printed circuit board. Accordingly, it is possible to arrange the cooling means on the printed circuit board, and the electric power supply circuit and the cooling means can be connected together merely by dipping the printed circuit board in a solder bath, making it possible to substantially eliminate the need of manually accomplishing electric connections. A similar description can equally apply to the electric connection between the magnetron and the inverter power source.
As a metal forming the metallic housing for the unitary structure, aluminum can be employed because of its excellent property of shielding noise. The employment of aluminum brings about an additional advantage in that the use of a noise filter hitherto necessitated in the magnetron can be eliminated. Also, since the inverter power supply, the magnetron and the cooling means are united together, the mounting of the metallic housing including the inverter power source, the magnetron and the cooling means can be accomplished by the use of an automated mounting machine with the consequence that manual labor can be reduced effectively.
The size of the unitary structure and, hence, the metallic housing, is preferred to be small and the component parts forming the magnetron and the inverter power supply are arranged at a high density within the metallic housing. For this purpose, the fan assembly for forcibly cooling those component parts must be small in size, but capable of being highly resistant to a loss of pressure. One example of the fan assembly includes a generally cylindrical fan assembly known as Silocco fan, and a compact D.C. motor capable of being driven at a high speed is suited as a drive motor for driving the Silocco fan.
Such a unitary structure for generating microwaves has some problems peculiar to it. For example, countermeasures against microwave hazards are not sufficiently taken. The unitary structure for generating the microwaves can be driven to generate the microwaves when electrically connected to a commercial electric power outlet. Also, the unitary structure includes the cooling means such as the fan assembly, the microwaves once generated therefrom can leak to the outside for a long time even though it is not fitted to a body of the high frequency heating apparatus, thereby posing a problem associated with microwave hazards.
Also, since the component parts for the magnetron and the inverter power supply are highly densely arranged to make the resultant unitary structure compact, some component parts operable with high and low voltages, respectively, tend to be shortcircuited by some reason.